The importance attached to reducing sound nuisance is well known both in the home and in industry, and although various means for combatting noise have been developed, the results obtained are not always satisfactory or they can be made satisfactory only at the cost of great difficulty. Thus, besides an initial approach which consists in limiting as much as possible the sound level emitted by a source such as an engine, a high speed flow of fluid, etc., proposals have been made to interpose protective walls between the source of sound and a region in which it is desired to reduce sound pressure, with the effectiveness of the protective walls increasing with increasing density of the material from which they are made. Nevertheless, good results can be obtained, e.g. in the building industry, only by using wall thicknesses that are technically and/or economically difficult to implement. Another approach then consists in performing acoustic correction by means of absorbent materials placed on a partition delimiting an enclosure to be protected so as to reduce as much as possible the reverberation of soundwaves on said partition. The sound pressure level reductions obtained in this way are of the order of 4 dB to 6 dB, and that does not make it possible to obtain an effect which is sufficient for significantly protecting enclosures exposed to sources of intense sound.
Using a different method, known as "active absorption", proposals have also been made to detect and analyze the soundwave emitted by a source of noise, and to cause said wave to disappear completely or partially by means of loudspeakers or analogous means disposed in the region to be protected and generating a soundwave in phase opposition with the incident source wave. Such a method is both complex and expensive, and consequently use thereof is limited to very specific cases where the regions to be protected are small in size and where the sound frequency ranges are not too large. That is why use is sometimes made of walls including air resonators of the Helmholtz resonator type as described for instance in British Patent Specification GB-A-2 027 255, or composite walls made up of volume-occupying elements secured to a support and that enter into resonance at predetermined frequencies as described, for instance, in German Patent Specification DE A-2 834 823. In order to work in the low frequency range (100 Hz to 300 Hz) walls of the first type (including Helmholtz resonators) require relatively large resonator volumes, while nevertheless limiting the acoustic corrections obtained in this range to the (necessarily small) ratio of the sum of the areas of the throats of the resonators disposed in the wall to the total area of said wall. Although the use of composite walls having volume-occupying elements turns out to be effective when the materials associated with the wall have large viscous friction (such as elastomers), this technique is nevertheless difficult to implement when a high degree of acoustic protection is sought over a wide range of sound frequencies.
The problem thus arises in the fields of acoustic protection, correction, and conditioning, of supplying a material and apparatus firstly enabling the drawbacks of known techniques to be mitigated, secondly being easy to implement and providing results that are satisfactory including over a wide audible frequency range, and finally having a cost that is economically acceptable.
A general object of the invention is to provide a material and apparatus incorporating such a material that enable this problem to be solved.
Another object of the invention is to provide a material and apparatus incorporating such a material which obtain very effective protection while using thin plates that are easy to make, that are suitable for being easily assembled, and for being cleaned, and in general, that are easily used by a nonspecialized user using means and tools that are simple and commonly available.